As a rule, the frequencies used in multifrequency codes are frequencies which, within limits of tolerance, are integral multiples of a single fundamental frequency. For instance, the frequencies used can be as follows:
______________________________________ f.sub.i = i .times. 100 Hz (N = 5) with i = 7 ; 9 ; 11 ; 13 ; 15 or f.sub.i = i .times. 40,6 Hz (N = 8) with i = 17 ; 19 ; 21 ; 23 ; 30 ; 33 ; 36 ; 40. ______________________________________
The invention uses digital generators or synthesizers of sinusoidal signals which are known in the prior art; their costruction will be briefly outlined. Such generators have a dead store containing, at addresses corresponding to recurrent values of an angular argument, the numerical values of the amplitudes of the samples of a sinusoidal function corresponding to the agruments, and provision for reading-out at a given timing, such provision being adapted to vary the stored digital values. Only the digital amplitudes of the samples corresponding to an argument range of (0 - .pi./2) need to be stored since the digital amplitudes of the (0 - 2 .pi. ) range can be deduced from the digital amplitudes of the (0 - .pi.2) range by the symmetries of the sinusoidal function. If, for instance, the quarter-period of the sinusoidal function is divided into P = 2.sup.x equal parts, so that the entire period is divided into 2.sup.(x.sup.+2) equal parts, the argument increment is .pi./2.sup.(x.sup.+1) and the addresses corresponding to the samples y.sub.0 at y.sub.[.sub.2.spsb.x .spsb.2.sub.- 1.sub.] of the function are:
______________________________________ addresses samples ______________________________________ 0 0 1 y.sub.1 ##STR1## ##STR2## ##STR3## ##STR4## ##STR5## ##STR6## ______________________________________
Clearly, therefore, when the sum of two addresses if 2.sup. x.sup.+1 (arguments whose sum is .pi.-i.c., supplementary)arguments), the samples are equal whereas when the sum of two addresses is 2.sup.x.sup.+2 (arguments whose sum is 2.pi.) the samples are opposite.